This invention relates to roadside beacon systems and, more particularly, to a novel roadside beacon system used for vehicle position calibration in a navigational system. After data on a departure point has been received, at least vehicle speed data and direction data are inputted to display the present position of the vehicle.
A so-called "navigational system" for automotive use has been proposed in the art, and details of such types of systems may be found in commonly assigned copending applications Ser. Nos. 026,403, 3-16-87; 026,359, 3-16-87, and Ser. No. 63383, filed 6/18/87. In such a navigational system, a small computer and a display unit are installed on a vehicle, road map data is read out of a memory comprising a compact disk or the like and displayed on the display unit, and vehicle speed data and direction data are received respectively from a vehicle speed sensor and a direction sensor so that the position of the vehicle may be calculated while the traveling direction thereof is determined. According to the results of this calculation and directional determination, a mark representing the vehicle is indicated on the road map displayed on the display unit to show the present position and traveling direction of the vehicle.
In this navigation system, the present position and traveling direction of the vehicle can be visually detected with ease, and therefore the vehicle driver can positively reach his or her destination without becoming lost.
However, this navigational system suffers from the following difficulties: Errors inherent in the vehicle speed sensor and the direction sensor are accumulated as the vehicle's traveling distance increases. That is, when the vehicle travels for a certain distance an accumulated error develops, the size of which depends on the degree of the errors of the vehicle speed sensor and the direction sensor, and variations in the environmental conditions of these sensors. The rate of error accumulation is not necessarily constant. Over time, the vehicle position displayed on the display unit is greatly deviated from the true vehicle position, so that the navigation system cannot perform its intrinsic function and the driver may lose his or her way.
In order to eliminate the above-described difficulty, a so-called "roadside beacon system" has been proposed in the art. In the roadside beacon system, roadside antennas are installed at certain distance intervals in a road traffic network which intervals are shorter than a distance for which errors accumulate to predetermined threshold values. A signal including position data and road direction data is radiated over a relatively small area from each of the roadside antennas thus installed, and the signal thus radiated is received by an antenna installed on the vehicle and inputted to the system computer, so that the position and traveling direction of the vehicle may be corrected according to the signal thus received.
When the roadside beacon system is employed, the display function can be performed with accurate position data and direction data with the accumulated errors can be held under predetermined threshold values at all times, and therefore the navigation system can perform its function correctly. Moreover, if the roadside antennas are installed at positions such as near railroads and railroad crossings where the on-board direction sensor is liable to be adversely affected in operation, then errors attributed to external factors can also be effectively corrected, advantageously.
In the above-described roadside beacon system, each of the roadside antennas is considerably high in directivity and radiates the signal including the position data and the road direction data at all times, and the vehicle receives the signal thus radiated only when passing through the area covered by the signal. If the area covered by the signal is increased, then the signal reception position with respect to the roadside antenna is greatly shifted, with the result that system correction or calibration is inaccurately performed.
This difficulty will be described in more detail. The fundamental function of the roadside beam system is to transmit a signal including position data and road direction data to vehicles having the navigation system. However, in order to effectively utilize the roadside beacon system, the following functions are also essential:
(1) Traffic data such as traffic congestion, road construction and the use of the roads around the roadside antenna are additionally provided to the navigation system so that the vehicle can travel smoothly.
(2) Detailed map data including the arrangement of houses near the roadside antenna are given so that the vehicle can readily reach its destination.
(3) Wide range road map data are transmitted to the navigation system to renew or supplement the road map displayed on the display unit, so that the vehicle can travel to far destinations.
In order to enable the roadside beacon system to perform these functions, it is necessary to increase the transmission band of the signal radiated from the roadside antenna and/or to increase the area covered by the signal.
However, if the transmission band of the signal and the area covered by the signal are increased as has been described above, the signal reception position with respect to the position of the roadside antenna is greatly shifted, as a result of which the calibration of the vehicle position, which is the original purpose of the system is not correctly carried out.
As the vehicle passes each roadside antenna, the positional relationships between the vehicle and buildings and other vehicles near the roadside antenna change with time. Therefore, as shown in FIG. 19, the signal radiated by the roadside antenna is received directly by the mobile antenna, and it is also received thereby after being reflected by the building, the road or another vehicle. These signals reaching the mobile antenna along different paths are different from one another in amplitude and in phase and are superposed in a cumulative or differential manner. Thus, the resultant signal is much different in signal strength distribution from the original signal transmitted by the roadside antenna as shown in FIG. 11 (i.e., a fading phenomenon attributable to the multi-path transmission occurs). Therefore, an error is caused when the vehicle position is calibrated according to the signal received. That is, as the resultant signal mentioned above provides a high level at a position far away from the roadside antenna, the position and traveling direction of the vehicle are undesirably calibrated when the high level is detected.
This difficulty may be overcome by a method in which a low-pass filter is used to eliminate the effect of the fading phenomenon on the signal strength distribution described above.
In general, however, the variation in signal strength due to the fading phenomenon occurs cyclically in a range from 10 Hz to 100 Hz. Therefore, the low-pass filter should have a cut-off frequency of several hertz (Hz). In order for a passive circuit to form a low-pass filter of such low cut-off frequency, it is necessary to use large inductive elements and large capacitive elements, which makes it difficult to miniaturize the device which is to be installed on the vehicle. If an active filter is used, the device can be miniaturized; however, the number of components is increased, the circuitry becomes intricate, and the manufacturing cost of the device is increased.
In order to overcome the above-described problem, there has been proposed a roadside beacon system wherein a signal modulated according to transmission data is radiated in the form of a split beam in order to enlarge the area, where the electric field strength is higher than a predetermined level, covered for data transmission; the electric field strength being abruptly lowered at a position directly confronting each roadside antenna. The transmission of necessary data is effected within the area where the electric field strength is higher than the predetermined level; and position calibration is effected by detecting a state wherein the electric field strength abruptly decreases.
This roadside beacon system enables enlargement of the area which is covered for data transmission but suffers from the problem that, since the electric field strength abruptly lowers at the position which directly confronts the roadside antenna, transmission of data cannot be effected in the state wherein it is possible to perform position calibration.